ORNL astrophysicist tells 'friends' about exploding stars

Bronson Messer, an astrophysicist at Oak Ridge National Laboratory, likes to answer questions and explain complex phenomena to lay audiences.

Carolyn Krause/The Oak Ridger

Bronson Messer, an astrophysicist at Oak Ridge National Laboratory, likes to answer questions and explain complex phenomena to lay audiences.

As a college senior he was a member of the University of Tennessee national championship team in a college quiz bowl competition. In 2003 he was a contestant on NBC’s “Jeopardy!” TV quiz show.

Now, Messer writes code and helps build three-dimensional models on an ORNL supercomputer to answer questions about supernovas. He and his colleagues rely on data from observations by astronomers and from labs around the world that detect signals from the cosmos, from neutrinos to gravitational waves.

Messer, who grew up in the heart of the Great Smoky Mountains and earned a Ph.D. in physics at UT, likes to answer questions from the various audiences he addresses.

Sometimes he's surprised by their reactions to what he says.

When he recently talked about his work to Friends of ORNL, he said that he has spoken to middle-school students and professionals in government about supernovas — exploding stars that have produced and disseminated most chemical elements throughout the universe, including those needed for life on the earth.

He gives his audiences this definition of a supernova from Encyclopedia Britannica: “a violently exploding star whose luminosity (intrinsic brightness) increases up to a billion times its normal level.”

He learned that middle-school students generally don’t know what the Encyclopedia Britannica is.

When he first compared the collapse and rebound of a supernova’s contents to squeezing and releasing a Nerf ball, he learned that many of today’s middle-school students don’t know what a Nerf ball is.

To explain how dense the iron core of a supernova becomes as the star collapses as a result of its own gravity and the exhaustion of hydrogen used in nuclear fusion to provide the pressure to resist collapse, Messer refers to a sugar cube — something most middle-school kids have never seen.

Eventually, he explained to FORNL, a massive star will shrink from a diameter of 60,000 kilometers (37,282 miles) to 60 km, or one-thousandth its original size, in a third of a second. A sugar cube with such a high density, he added, would weigh as much as all the people on our planet.

Messer surprises most people in his audiences with information about Betelgeuse (pronounced “Beetle Juice”). It’s the eighth brightest star in our sky and appears red if you focus on the constellation Orion, especially Orion’s shoulder.

Betelgeuse has 20 times the mass of the sun but is 640 light years away, meaning that light emitted by the star will take 640 years to reach our planet.

“Betelgeuse, which is 10 million years old, could go supernova anytime or up to 100,000 years from now,” Messer said. If it is or becomes a supernova, it will be visible during the day for a month and a half and possibly brighter than the full moon at night for two-thirds of a year, making it the second brightest light in the sky after the sun.

When Messer gave his talk about this super red giant to a group from the U.S. Department of Defense, “several in the audience started freaking out,” he said. “I told them a Betelguese supernova could be brighter than a full moon at night.”

One DOD official exclaimed, “That will change night-time warfare!”

Messer has been working on the CHIMERA nuclear astrophysics codes that have been run at ORNL on the Titan, the world’s fourth fastest supercomputer for unclassified science. It can perform up to 27 million billion calculations per second using equations and observational data.

The 3D models simulate the collapse of a dying massive star’s core, its shock wave during rebound and resultant explosion. They also simulate the transport of energy from the star’s center to the surface by photons of light (1 percent) and by nearly massless and barely detectible neutrinos (99 percent).

Early in the 2020s, ORNL will have an exascale supercomputer named Summit. It will perform calculations five times faster than Titan. Summit will have the capability to make a billion billion calculations (addition, subtraction, multiplication, division) per second.

“We are trying to find out where the chemical elements come from,” Messer said. “We know that hydrogen, helium, and a little lithium and beryllium were produced in the Big Bang but all heavier elements were cooked in explosions in dying stars.”

Messer said that Summit will enable his group to simulate magneto-hydrodynamics, the magnetic properties of electrically conducting fluids in collapsing stars. In addition, Summit will help astrophysicists identify and track the production of various elements and their isotopes during simulated explosions in core-collapse supernovas.

ORNL researchers write nuclear astrophysics codes with groups at the Department of Energy’s Lawrence Berkeley and Argonne national labs and at Stony Brook University in New York.

“We want to understand the behavior of extremely dense mass, the properties of neutrinos and sources of gravitational waves, one of which was detected in the past 18 months,” Messer added, noting that gravitational wave astronomy is the latest new field.

The goal of the astrophysicists is to build 3D models that make predictions about the behavior of matter in supernovas that nearly match the data collected by sophisticated instruments. The models will likely answer some questions and raise new ones.

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